There has long been a desire to formulate a curing agent which is immediately compatible with conventional epoxy resins, especially at low curing temperatures, and which is sufficiently reactive with epoxy resins that the system will cure in a wide range of temperatures, even as low as 4.4.degree. C., within a 24 hour period in the absence of external accelarators if possible.
Typical amine curing agents, whether aliphatic, aromatic, or adducts with epoxy resins, have terminated with at least one primary amine group. The primary amines are notorious for producing the undesired side effect of blooming or hazing in the cured product. This phenomena is thought to result from the reaction between the highly reactive primary amine groups in the curing agent with atmospheric carbon dioxide and moisture to produce carbamates, resulting in scission of the curing agent polymer chain. Blooming or hazing is more likely to be encountered when the curing agent is stored for a lengthy period of time, and applied in low temperature or high humidity environments.
To some extent, this problem can be ameliorated by reacting out many of the primary amine hydrogens. The drawback to this approach in the past has been that the reactivity of the curing agent was impaired because secondary amines are less reactive than the primary amines, such that accelerators had to be used to obtain adequate cure times, especially at low curing temperatures. Furthermore, many of the amine curing agent adducts whose primary amine groups were converted to secondary amine groups are poorly compatible with epoxy resin.
It would be desirable to have a curing agent composition for curing epoxy resins whose primary amine groups are converted to secondary amine groups, yet is reactive enough to cure epoxy resins without external catalysts/accelerators in a wide range of curing temperatures and which is immediately compatible with a standard diglycidyl ether of Bisphenol-A.